In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after release 12 (see Non-Patent Document 1).
In the D2D communication, a plurality of user terminals proximal to one another are able to perform direct communication with each other in the state where a radio connection with a network is established (in the state where synchronization is achieved). In addition, the D2D communication is also called Proximity Service communication.